Add overwrite message support to the ValidationResult

[rick-nu]

What

This PR adds support to overwrite validation messages on the validation result. This could come in handy overwriting validation messages in a view (when using MVC).

How to test

1. See that you can overwrite validation messages in the validation result, including message variables
2. See that you can no longer overwrite specific messages on the validator
3. See that all tests pass and coverage/quality is still 100%

ToDo

• Update documentation
• Remove ->overwriteMessages() from the Validator class.

[rick-nu]

This is no longer a use-case, since the messages will always be overwritten from the validation result. This decreases the complexity, which is another plus of this feature.

[rick-nu]

Ok, I have an issue with this for the following feature: #80. Now the specific messages are set on the validation result. But in the ->output() function I would like to receive all possible and final validation messages. I need to find a way to set those so that they are available for that function as well. I can't make it part of the ValidationResult, because the validator will have to run ->validate() before you could do a ->output().

[rick-nu]

After a log discussing with @rskuipers we decided that overwriting the messages in a template file is not going to work with functionality like output. Therefor I decided to close this PR, and start to work on the output functionality instead. The validation result class is still a nice improvement, but overwriting the messages still needs to be done on validator level.